It has been customary in the electronic article surveillance industry to apply EAS markers to articles of merchandise. Detection equipment is positioned at store exits to detect attempts to remove goods with active markers from the store premises, and to generate an alarm in such cases.
When a customer presents an article for payment at a checkout counter, a checkout clerk deactivates the marker by using a deactivation device provided to deactivate the marker.
One type of EAS system is referred to as a harmonic system because it is based on the principle that a magnetic material passing through a magnetic field having a selected frequency disturbs the field and produces harmonic perturbations of the selected frequency. The detection system is tuned to recognize certain harmonic frequencies and, if present, causes an alarm. The harmonic frequencies generated are a function of the degree of non-linearity of the hysteresis loop of the magnetic material. An example of a harmonic EAS system is disclosed in U.S. Pat. No. 4,660,025, which is commonly assigned with the present application.
Another type of EAS system is known as a magnetomechanical system, and utilizes markers that include a magnetostrictive element. A system of this type is disclosed in U.S. Pat. No. 4,510,489. Markers used in magnetomechanical systems are formed of a ribbon-shaped length of a magnetostrictive amorphous material contained in an elongated housing in proximity to a bias magnetic element. The magnetostrictive element is fabricated such that it is resonant at a predetermined frequency when the bias element has been magnetized to a certain level. At the interrogation zone, a suitable oscillator provides an AC magnetic field at the predetermined frequency, and the marker mechanically resonates at this frequency upon exposure to the field when the bias element has been magnetized to a certain level along the length of the bias element. In a widely-used kind of magnetomechanical EAS system, the interrogation field is provided in pulses or bursts. A marker present in the interrogation field is excited by each burst, and after each burst is over, the marker undergoes a damped mechanical oscillation. The resulting signal radiated by the marker is detected by detecting circuitry which is synchronized with the interrogation circuit and arranged to be active during the quiet periods after bursts.
In a magnetomechanical EAS marker, the bias element functions as a control element. If it is desired to deactivate the magnetomechanical marker, the magnetic condition of the bias element is changed so that the bias element no longer provides the bias field required for the marker to resonate at the predetermined frequency of the detection equipment.
According to one known technique for deactivating a magnetomechanical marker, the bias element is degaussed by exposure to an AC magnetic field. According to another known technique, the magnetomechanical marker is brought into contact with, or very close to, an array of small permanent magnets arranged with alternating polarities. This breaks up the magnetization of the bias element along its length so that it no longer provides the bias field required to condition the magnetostrictive element for mechanical resonance.
It is also possible to deactivate a magnetomechanical marker by changing the orientation of magnetization of the bias element, so that the polarity of magnetization is orientated across the width of the bias element rather than along its length.
It is also known to provide control elements for harmonic markers. For example, a sequence of magnetic elements is mounted along the length of the harmonic marker. When these elements are in a demagnetized condition, the marker is activated and will produce harmonic perturbations in response to the interrogation signal. To deactivate the harmonic marker, the control elements are magnetized by exposing the marker to a strong DC magnetic field, generated, for example, by a permanent magnet or a DC-driven electromagnet. When the control elements are magnetized, the marker is prevented from causing the harmonic perturbations in the interrogation field.
As retail stores and shopping malls become larger, it is increasingly likely that both harmonic and magnetomechanical EAS systems will be in use in the same facility. For example, one department of a store may employ a magnetomechanical EAS system while another department employs a harmonic system. If a common checkout counter is shared by both departments, it would be necessary to provide at the checkout counter facilities for deactivating both types of marker. It could be contemplated to provide at the checkout counter a separate deactivation device for each type of marker, but this approach would be expensive and would take up too much space at the counter. It could also be attempted to use a single device of the type which generates a DC magnetic field to deactivate both types of marker, by magnetizing the control elements in the case of the harmonic markers, and by producing a widthwise magnetization in the control element of the magnetomechanical marker. However, such a device is not likely to provide reliable deactivation of the magnetomechanical marker because of difficulty in assuring that the field is applied in the correct orientation relative to the magnetomechanical marker. Also, for a magnetomechanical marker having a low-coercivity bias element, as disclosed in U.S. Pat. No. 5,729,200, it has been found that widthwise magnetization of the bias element is difficult to achieve, so that deactivation by application of a DC magnetic field is problematic. Moreover, the DC-field type of deactivation device would require both types of marker essentially to be brought into contact with the deactivation device, and is not suitable for the more desirable and efficient practice of "distance deactivation".
Another possible solution would be a deactivation device of the type which employs an alternating polarity array of permanent magnets. However, again this is a contact deactivation type of device, and although reliable deactivation of magnetomechanical markers can be expected, there would be a substantial possibility of failing to reliably deactivate harmonic type markers with this kind of device.